Recently, it has been reported that doped semiconductor nanoparticles can yield both high luminescence efficiencies and a spectacular lifetime shortening, which suggests that doped semiconductor nanoparticles form a new class of luminescent materials for various applications. From lifetime measurements and time-resolved spectroscopy we conclude that the ${\mathrm{Mn}}^{2+}$ emission does not show a spectacular shortening of the decay time upon decreasing particle size as reported earlier. The luminescence of nanocrystalline ${\mathrm{Z}\mathrm{n}\mathrm{S}:\mathrm{M}\mathrm{n}}^{2+}$ indeed has a short decay time (\ensuremath{\sim}100 ns), but also shows a long ms range decay time. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the ${}^{4}{\mathrm{T}}_{1}{\ensuremath{-}}^{6}{\mathrm{A}}_{1}$ transition of the ${\mathrm{Mn}}^{2+}$ impurity as suggested by other authors. The ${}^{4}{\mathrm{T}}_{1}{\ensuremath{-}}^{6}{\mathrm{A}}_{1}$ transition of the ${\mathrm{Mn}}^{2+}$ has a ``normal'' decay of about 1.9 ms. Based on our observations, we conclude that doped semiconductor nanoparticles do not form a new class of luminescent materials, combining a high efficiency with a short (ns) decay time.